The present invention relates to a novel diaphragm, particularly for use in electrolysis cells, and to a novel method of making said diaphragm. The invention is an improvement in the diaphragm disclosed in our parent U.S. patent application, Ser. No. 469,808, filed May 14, 1974, commonly owned, now U.S. Pat. No. 3,980,613.
Our parent copending application, Ser. No. 469,808, describes a method of making porous diaphragms of asbestos which is deposited and consolidated by a fluorinated polymeric resin, characterized in that a stable, homogeneous suspension is formed by adding a fluorinated polymeric resin latex and a pore-forming agent to a suspension of asbestos fibers in water, in the presence of a sulphonic anionic surfactant, followed by placing said suspension in a filtration mold, drying and fritting the resulting casting at a temperature above the crystalline melting point of said fluorinated polymeric resin, and with the pore-forming agent being finally eliminated by decomposition or chemical action.
One of the requirements of the diaphragm of an electrolytic cell is that it behave as a porous medium: (1) on the one hand, it allows current to flow with a slight ohmic drop, and on the other hand, (2) it permits an even flow of electrolyte from one compartment of an electrolytic cell to the other. There is, consequently, a combination of mechanical, electrical and hydraulic conditions which are all the more critical now that electrolytic cells are required to function with high current density, subject to tolerating prohibitive ohmic drops. The properties required are rather contradictory. Thus, from a mechanical point of view, the diaphragm must have a well-defined, relatively permanent geometry or shape. The diaphragm must not be subject to swelling and being permanently deformed during use. In other words, such a diaphragm must have good mechanical strength. However, it must also have good wettability properties, enabling gases to be released and electrolyte to be circulated, while preventing diffusion of hydroxide ions in the opposite direction to the liquid flow. Such diffusion is responsible for the formation of chlorate which leads to a drop in yield.
To express the situation differently, a diaphragm for electrolysis must have low relative resistance. Relative resistance is understood as being the quotient of the resistance of a medium consisting of the diaphragm impregnated with electrolyte and the resistance of the same medium consisting of electrolyte. It has been observed that the relative resistance is bound up with the porosity of the diaphragm, but also with the shape of the flow passages.
Attempts have therefore been made to associate a plurality of elements so as to meet all the above requirements simultaneously. Thus, U.S. Pat. No. 3,694,281 proposes forming a mechanical support consisting of plastic filaments and arranged on at least one face of the diaphragm, or making a laminate with threads, fibers or particles inserted in the interface between the asbestos and the substrate.
French Pat. No. 2,170,247 proposes a simple arrangement which makes use of a support consisting of a cloth, e.g., of polypropylene.
Belgain Pat. No. 814,510 proposes improving the rigidity of a diaphragm by applying the diaphragm to at least one sheet of chrysotile asbestos. In a special embodiment of the diaphragm, the sheet comprising the diaphragm is sandwiched between and bonded to two sheets of chrysotile asbestos.
However, the main difficulty encountered in providing a preferred diaphragm is to maintain the advantages of an open structure as provided by the diaphragm produced in accordance with our prior copending patent application, Ser. No. 469,808, while improving its mechanical properties. When such diaphragms are used industrially, they are in fact found to have the disadvantage of showing structural deformation, particularly localized loss of cohesion, and to result in performance which cannot be perfectly reproduced.
It has been found that this defect can be avoided if the diaphragm obtained according to our prior copending application, Ser. No. 469,808, is associated with a reinforcing element, by integrating it with the anode face of the porous membrane, the reinforcing element having a structure which is at least as open, relative to the passage of liquid during electrolysis, as the porous membrane.
It is, accordingly, an object of the present invention to provide an improved diaphragm over that disclosed in our prior copending application, Ser. No. 469,808.
It is also an object of the present invention to provide an improved process for producing electrolytic cell diaphragms which are superior to those disclosed in our said prior copending application, Ser. No. 469,808.
Other objects will be apparent to those skilled in the art from the present specification.